1. Field of the Invention
The invention relates to a method of detecting an off-balance condition of a clothes load in a washing machine.
2. Description of the Related Art
Various appliances, such as automatic washing machines, automatic dryers, centrifugal liquid extractors, etc., utilize a rotating tub, basket, or other vessel holding a load of material that can be evenly or unevenly distributed within the vessel. The condition of having the load unevenly distributed, or out of balance, creates a situation where the center of mass of the rotating vessel does not correspond to the rotational axis of the vessel. In a washing machine, as the speed of the vessel increases during a spin extraction cycle, an unbalanced load can lead to different types of phenomena, including rocking of the vessel relative to the cabinet within which it is supported and hitting of the cabinet by the vessel, as will be described in further detail below. This leads to the generation of high loads and severe vibration of the vessel. Such severe vibration can cause movement of the appliance across the floor or other supporting surface.
As illustrated in an exemplary schematic vertical axis washing machine 100 of FIG. 1, the washing machine 100 typically comprises an imperforate tub 102 mounted within a cabinet 104 and a perforated wash basket 106 mounted within the tub 102 and rotatable relative to the tub 102. The wash basket 106 defines a wash chamber 108 that can receive a load of clothes to be subjected to various wash, rinse, and spin cycles, as is well-known in the washing machine art. A motor 110 operably coupled to the wash basket 106, an agitator 112 mounted in the wash basket 106, and a controller 116 rotates the wash basket 106 and/or the agitator 112 according to the wash, rinse, and spin cycles executed by the controller 116.
The tub 102 and the wash basket 106 are suspended within the cabinet 104 by a suspension system 114, which dampens some vibratory movement of the tub 102 and the wash basket 106. As a result of this suspended configuration, the suspended mass comprising the tub 102, the wash basket 106, and the clothes load in the wash basket 106, has six degrees of freedom; the suspended mass can translate along an x-axis (side-to-side movement), a y-axis (front-to-back movement), and a z-axis (up-and-down movement) and can rotate about the x-, y-, and z-axes, which are illustrated in FIG. 2.
During the spin cycles, the motor 110 increases the rotational speed of the wash basket 106 according to a spin profile, which can comprise various speed ramps and speed plateaus. As the speed increases, the suspended mass passes through natural frequencies corresponding to the six degrees of freedom. At these natural frequencies, the suspended mass has a natural tendency to move according to the corresponding degree of freedom, and this tendency is increased dramatically when the clothes load is off-balance. Thus, when the suspended mass passes through x-axis and y-axis translational natural frequencies, the suspended mass with an off-balance load can swing side-to-side and front-to-back, much like a pendulum, and hit the sides of the cabinet 104. Similarly, when the suspended mass passes through the z-axis translational natural frequency, the suspended mass with an off-balance load has a tendency to move up-and-down and thereby hit the top of the cabinet 104 and/or bottom out the suspension system 114. Finally, when the suspended mass passes through the rotational natural frequencies, the suspended mass with an off-balance load has a tendency to rock within the cabinet 104.
Various attempts have been provided in the prior art to provide mechanical arrangements, such as paddle switches, to detect the presence of an off-balance load by physically detecting when the vessel approaches or hits the cabinet. However, mechanical switches can be costly, are not robust to levelness, and might not distinguish between potentially acceptable light cabinet hitting and unacceptable heavy cabinet hitting. As gaps between the vessel and the cabinet of washing machines continue to decrease as vessel capacity increases, the ability to distinguish between light and heavy cabinet hits becomes more essential.
Approaches have also been disclosed in the prior art for detecting a load imbalance by monitoring variation of an output, such as motor current or voltage signal, of an operational component of the washing machine to eliminate mechanical switches and reduce cost. Often, the output is processed in some manner and then compared to a predetermined threshold for determining whether an imbalance is present. Depending on the output utilized, such methods are usually only suitable for particular speeds during a spin cycle and can be unreliable, even at the suitable speeds. Additionally, if the methods are suitable at spin speeds corresponding to only one or some of the translational or rotational natural frequencies, then off-balance loads that are not detected by or deemed acceptable by the method can potentially cause damage to the washing machine when they reach and pass through the other natural frequencies at higher spin speeds.